Under the NIGMS shared instrumentation program we request funds for an automated, high speed, flat-bed, scanning microdensitometer with photo-writing capability (film scanner-writer). The Johns Hopkins Medical Institutions have a long-standing commitment to ultra-structural studies of biomedical importance. The requested instrument will benefit many of the investigators engaged in these studies, and will provide another strand in an already extensive web of collaboration. Furthermore, it will help to increase intellectual and technical exchange between the different ultrastructural research groups at this Institution. Within the group of core users the applications will include extraction of integrated Bragg intensities from X-ray diffraction photographs of crystals of various immunoglobin fragments, mitochondrial ATPase F1 fragment, SV-40 and polyoma viruses; quantitation of fluorescence light micrographs, and the display of images created by three-dimensional reconstruction from nuclear magnetic resonance. Applications suggested by potential users include image processing for electron microscopy, automation of bacteriophage and hemolytic plaque counting, ultracentrifuge plate quantitation, and karyotyping. No film scanner exists in Baltimore at this time. The nearest automatic film scanner is at the Applied Physics Laboratory of JHU. It is 25 miles away, of difficult access for distance and security reasons and incapable of handling rigid (glass) plates. The Principal Investigator and other core investigators have had experience with microdensitometer instruments similar to the one proposed here, an experience which provides an adequate introduction to the intended use. If purchased through this application, the instrument will be introduced and made available to all interested parties. Institutional support exists and adequate administrative arrangements will be made for the use of this proposed interdepartmental facility.